1. Field of the Invention
The present invention relates to an irradiation method of laser light and an irradiation device for conducting the method (the device including a laser oscillation device and an optical system for guiding a laser light outputted to an object to be irradiated). Further, the present invention relates to a method for manufacturing a semiconductor device including steps of crystallization, activation, heating and the like of a semiconductor film. Note that the term semiconductor device mentioned here includes electro-optical devices such as a liquid crystal display device, a light emitting device and an electronic device including the electro-optical device as a component.
2. Description of the Related Art
In recent years, an extensive study has been made on a technique in which an amorphous semiconductor film formed on an insulating substrate made of glass or the like is crystallized so that a semiconductor film having crystal structure (hereafter referred to as crystalline semiconductor film) is obtained. As the methods of crystallization such as a thermal annealing method using furnace annealing, a rapid thermal annealing method (RTA method), a laser annealing method and the like were examined. Anyone thereof or combining two or more methods thereof can be carried out for crystallization.
In comparison with an amorphous semiconductor, a crystalline semiconductor film has extremely high mobility. Since thus, the crystalline semiconductor film is used to form a thin film transistor (referred to as TFT), for example, the TFT can be widely used in an active matrix liquid crystal display device in which TFTs for pixel portion, TFTs for pixel portion and TFT for driver circuit are formed on one glass substrate.
Generally, in order to crystallize an amorphous semiconductor in annealing furnace, a thermal treatment at 600° C. or more for 10 hours or more is required. A quart is an applicable material of substrate for this crystallization, but the quart substrate is too expensive in price to be manufactured especially in a large area. However, manufacturing the substrate in a large area is given as a way to improve the productivity efficiency, that is why a study has been made on formation of a semiconductor film on a glass substrate which can be manufactured easily in a large area at a low price. It is expected that a substrate in which a length of one side exceeds 1 m will be also used in recent years.
As an example, a method of thermal crystallization by using metal elements disclosed in Japanese Patent Application Laid Open No. 7-183540 enable the crystallization temperature which was a conventional problem to be realized at a low temperature. The crystalline semiconductor film can be formed by this method in which a small amount of an element such as nickel, palladium and lead is added to an amorphous semiconductor film, then the amorphous semiconductor film is heated for four hours at 550° C. 550° C. is a temperature under the strain point of a glass substrate, therefore, there is no worry about a distortion caused thereof.
On the other hand, since the laser annealing method can deliver high energy only to the semiconductor film without substantially increasing the temperature in substrate, the laser annealing technology comes under spotlight by its appliance in a glass substrate with a low strain point as a matter of course, and a plastic substrate, etc.
An example of the laser annealing method is a method for forming pulse laser beam from an excimer laser or the like by an optical system such that it becomes a square spot of several cm or a linear shape of 100 mm or more in length on a surface being illuminated, and relatively shifting an illumination position of the laser beam with respect to the surface being illuminated to conduct annealing. The “linear shape” described here means not a “line” in the strict sense but a rectangle (or a prolate ellipsoid shape or a shape similar thereto) having a high aspect ratio. For example, although, it indicates a shape having an aspect ratio of 2 or more (preferably, 10 to 100), it doesn't make any difference from that a shape at a surface being illuminated is being contained in the laser light having rectangular shape (rectangular shape beam). Note that the linear shape is used to obtain an energy density required for annealing an object sufficiently to be illuminated. Thus, if sufficient annealing is conducted for the object to be illuminated, it may be a rectangular shape and a tabletop shape.
However, a crystalline semiconductor film formed by subjecting an amorphous semiconductor film to laser annealing includes a collection of a plurality of crystal grains, and the position and size of the crystal grains are random. TFTs are formed on a glass substrate by patterning the crystalline semiconductor layer in an island shape for device separation. In this case, the position and size of crystal grains cannot be specified. In comparison with the inner of crystal grains, the interface of crystal grains has an infinite number of a recombination centers or a trapping centers caused by an amorphous structure, a crystal defect, and the like. If the carriers are trapped in trapping centers, potential at a grain boundary will be increased and become barriers to carriers, it is known that current transporting characteristics of carriers will be degraded caused by this. However, it is almost impossible to form a channel formation region by using a single crystal semiconductor film while avoiding the influence of a crystal boundary, although crystal characteristics of semiconductor film of channel formation region have a serious effect on the TFT characteristics.
There is a crystal growth technology reported. In the technology, when a CW laser is illuminated on a semiconductor film with the continuous wave (CW) laser scanning in one direction, crystal grains grow connected in the scanning direction thereof, resulting in forming an infinite number of single crystal grains elongated in that direction. For example, this technology is reported in “Ultra-high Performance Poly-Si TFTs on a Glass by a Stable Scanning CW Laser Lateral Crystallization”, A. Hara, F. Takeuchi, M. Takei, K. Yoshino, K. Suga and N. Sasaki, AMLCD '01 Tech. Dig., 2001, pp. 227-230.
It is considered that when this method is applied, a semiconductor film that has no grain boundary at least in a channel direction of a TFT can be formed.